Ceramics produced by sintering a mixture principally consisting of ZnO with an amount of additive added thereto is known to show a superior voltage nonlinearity. Therefore, this mixture has been widely used in the industry for varistors for controlling an abnormal voltage (surge) in electric circuits.
The voltage nonlinearity of a ZnO varistor is due to a Schottky barrier formed on grain boundaries of the ZnO grains. In a practical varistor, its varistor voltage per layer of grain boundaries formed by combining the ZnO grains is almost constant independent of the crystal particle size. The value of the varistor voltage is about 2 volts per layer of grain boundaries. The varistor voltage is defined as the voltage across its terminals when a current of 1 mA is caused to flow into a varistor and its level is usually expressed as V.sub.1mA. The varistor voltage of a voltage-nonlinear resistor is therefore determined by the number of grain boundary layers existing between electrodes which are placed on a sintered body of ZnO. If the voltage-nonlinear resistor to be used for a low-voltage circuit, it is necessary to make the thickness of the element thin or to make the ZnO grain size sufficiently large.
For example, when used for a 12 V DC circuit, generally, a ZnO varistor having a varistor voltage of 22 V is used in view of fluctuations of the circuit voltage. In this case, however, the varistor can have only 11 layers of grain boundaries existing between its terminal electrodes of the resistive element since the varistor voltage per layer of grain boundary is about 2 V as described above.
On the other hand, a usual fabrication method produces a ZnO sintered body of the varistor with a grain size of 10-20 .mu.m. It is therefore necessary to select the thickness of the element to be 0.1-0.2 mm in order to obtain the varistor voltage of about 22 V. However, a sintered body for such a ZnO varistor of 0.1-0.2 mm thickness has low mechanical strength, which thereby causes a problem in that a crack may be generated in production of the sintered body or the like. Accordingly, such a method which relies on the thinness of the element is not practical.
In order to solve the problem, there has been disclosed in Japanese Patent Examined Publication No. 56-11203 a skillful method in which a small amount of ZnO single crystals of much larger grain size than that of raw material ZnO powder is added to the ZnO powder so that grain growth is accelerated with the ZnO single crystals acting as seeds (hereinafter referred to as "seed grains"). FIG. 1 shows a basic process flow of this method. The method comprises the steps of mixing the varistor powder and the seed grains molding the mixture, and then sintering the molded mixture.
When the mixture of seed grains and varistor powder is sintered, grain growth is accelerated with the seed grains as crystal growth seeds because of the difference in surface, energy. As a result, extremely larger crystal grains can be obtained in comparison with those in the case of addition of no seed grains. FIG. 2 is a diagram typically illustrating such a situation. In FIG. 2 are shown a raw material powder 1, and crystal grains 2 in the sintered body. FIG. 2 shows a situation in a conventional method in which no seed grains are added. In this situation, the grain size is 50 .mu.m at the largest even if the sintering temperature is made high or the sintering time is prolonged. If sintering is thus made at a high temperature and for a long time, a nonlinear voltage coefficient .alpha. of the element is extremely lowered because of evaporation of the additive and so on so that the element is not suitable for practical use. On the other hand, FIG. 3 is a diagram typically illustrating a situation in the case where seed grains are added. Each crystal grain, grows from a seed grain 3 into a giant grain 4. According to this method, each crystal grain 4 grows to 100-200 .mu.m in its size so that it is possible to lower its varistor voltage per mm of element thickness to 20 V/mm or less.
In order to produce seed grains used for accelerating grain growth, the following methods are generally used. (1) After molding a mixture of powder in which a small amount of a Ba or Sr compound is added to the ZnO powder, the molded mixture is sintered and the thus obtained sintered body is hydrolyzed. (2) After molding a mixture of powder in which a grain growth accelerator such as Bi.sub.2 O.sub.3, a rare earth compound or the like is added to the ZnO powder, the molded mixture is sintered and the thus obtained sintered body is ground. (3) ZnO single crystals are directly formed by using a vapor-phase epitaxial method.
Of the above seed grain production methods, the first method (1) has been most often used because the Ba or Sr compound used as a grain growth accelerator can be removed by hydrolysis, and the additive and the seed grain size can be easily controlled. FIG. 4 shows a process flow chart of a prior art ZnO varistor production method incorporating this seed grain production process. It will be apparent from FIG. 4 that the seed grain production process require many steps.
There are however the following problems in the ZnO varistor production method including the above-mentioned prior art seed grain production process. Therefore, it has not always been a satisfactory method because of variations in product characteristics, in production, cost, and so on.
Since the seed grains are not spherical in shape, the seed grains are not equal in grain size after sintering and variations occur in electrical characteristics.
Because of large variations in the seed grain size, the yield of usable seed grains is small.
Much time is spent for the hydrolysis step in making the sintered body into single crystals.
Lastly, it is necessary to provide a separate line for producing the seed grains.